Nowadays, rare earth magnets as typified by Nd magnets are widely used in various motors, sensors and other components mounted in hard disk drives, air conditioners, hybrid vehicles, and the like. As to the rare earth elements from which rare earth magnets are manufactured, their resources are found only in limited countries. A resource crisis is exclaimed because it is expected that the demand will exceed the supply in the near future. There is a strong demand for the reuse of magnet powder, debris and scraps associated with the manufacture of rare earth magnets, the recovery by recycling of rare earth elements from municipal wastes, and the research and development of new rare earth mineral deposits.
Rare earth elements for use in rare earth magnets include cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), terbium (Tb), dysprosium (Dy), and the like. Known techniques for the separation of these rare earth elements include ion-exchange resins (liquid to solid extraction) and solvent extraction (liquid to liquid extraction). For the purification and separation of rare earth elements in an industrial manner, the solvent extraction technique is often used because consecutive steps enable efficient large volume processing. In the solvent extraction technique, an aqueous phase in the form of an aqueous solution containing a metal element to be separated is contacted with an organic phase containing an extractant for extracting the specific metal element and an organic solvent for diluting the extractant, for thereby extracting the metal element with the extractant. The metal element is separated in this way.
A variety of extractants are used in the art, for example, tributyl phosphate (TBP), carboxylic acid (Versatic acid 10), phosphoric acid esters, phosphonic acid and phosphinic acid compounds. An exemplary phosphoric acid ester is di-2-ethylhexylphosphoric acid (D2EHPA), a typical phosphonic acid compound is 2-ethylhexylphosphoric acid mono-2-ethylhexyl ester (PC-88A by Daihachi Chemical Industry Co., Ltd.), and bis(2,4,4-trimethylpentyl)phosphoric acid (Cyanex 272 by American Cyanamid) is commercially available as a phosphinic acid compound. They are commonly used in the industry.
The separation efficiency of the solvent extraction technique depends on the ability of an extractant, especially its separation factor. The greater the separation factor, the higher becomes the separation efficiency of the solvent extraction technique, which leads to simplification of separation steps and downsizing of the separation system, and eventually to an improvement of the process efficiency and a cost reduction. Inversely, if the separation factor is lower, the separation process becomes more complicated and the separation system becomes of larger size.
Of the commercially available extractants, PC-88A which is known to have a high separation factor for rare earth elements has a low separation factor between neighboring elements, for example, a separation factor of less than 2, specifically about 1.4 between neodymium and praseodymium which are believed most difficult to separate among the rare earth elements. The separation factor of this order is not sufficient to facilitate separation between neodymium and praseodymium. To separate them at an acceptable purity, a large size system is necessary and capital-intensive. For the purpose of purifying and separating such rare earth elements, it is desired to have an extractant having a higher separation factor than ever and an extraction/separation method using the same.
One extractant known to have a high separation factor for rare earth elements, especially light rare earth elements: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), and samarium (Sm) is a diglycolamic acid as disclosed in JP-A 2007-327085. Solvent extraction using this extractant makes more efficient the process of extracting and separating rare earth elements, especially light rare earth elements. The process of extracting and separating rare earth elements using this extractant demonstrated better results in an experiment, which was conducted at an extractant (diglycolamic acid) concentration of 0.03 M and a rare earth element concentration of 0.1 mM. The latter concentration corresponds to about 1/1,000 of the rare earth element concentration found in actual rare earth element separation processes. The process cannot comply with the treatment of high concentration aqueous rare earth solutions. Using the extractant in a concentration which is 100 to 1,000 times the rare earth concentration is not industrially acceptable because the extraction/separation process must deal with a large solution volume which requires a larger size of separation system and hence an increased expense. Although the process of extracting and separating rare earth elements using the diglycolamic acid demonstrated better results in an experiment, various conditions under which extraction may be practically effected with this extractant in the industry have not yet been determined because this extractant differs in chemical properties from the commercially available extractants, D2EHPA, PC-88A and Cyanex 272.